This invention relates to linear copolyesters that contain both carbonate groups and carboxylate groups in a linear chain.
Polycarbonate resins are known to be tough and rigid and have moderately high softening temperatures. Of particular interest are the polycarbonates of bisphenol-A diols as described in U.S. Pat. No. 3,028,365. On the other hand, polyesters such as those derived from terephthalic acid, isophthalic acid and/or 1,4-butanediol are well known as molding resins having high softening temperatures but poor impact resistances.
In the past, it has been a practice to make random linear copolymers containing ester and carbonate linkages in order to obtain polymers having heat distortion temperatures generally higher than those characteristic of polycarbonates. See, for example, U.S. Pat. Nos. 3,169,121; 3,549,570; 3,053,810; 3,030,331 and 3,220,976. Unfortunately, however, the desired increase in heat distortion is often not as high as needed for many applications. More importantly, any increase in heat distortion is achieved only by sacrificing almost all of the high impact resistance that is characteristic of polycarbonate resins.
More recently, as described in U.S. Pat. Nos. 4,156,069 and 4,105,633, it has been found that, by alternating or ordering the ester and carbonate linkages in the copolyestercarbonate polymer molecule, improved thermal resistance is achieved without a corresponding sacrifice of physical strength. However, such ordered copolyestercarbonates do not exhibit the degree of processability desired for many molding applications.
In view of the aforementioned deficiencies of conventional polyesters, polycarbonates and random, as well as ordered, copolymers thereof, it would be highly desirable to provide a polymer of the same or similar monomeric materials wherein improved processability is obtained without sacrificing the impact resistance and heat resistance characteristics of conventional ordered copolyestercarbonates.